1. Field of the Invention
The present invention relates to a thermal head capable of printing two lines at the same time using two lines of heating elements, and to a thermal head capable of performing preheating using one of two lines of heating elements while performing printing using the other one of two lines of heating elements thereby achieving a high-speed printing operation.
The present invention also relates to a thermal head controller, and more particularly, to a thermal head controller for controlling a thermal head including a preheating heater and a printing heater.
2. Descripiton of the Related Art
FIG. 23 illustrates a thermal head disclosed in Japanese Unexamined Patent Application Publication No. 64-58566, wherein FIG. 23A is a top view of the thermal head and FIG. 23B is a cross-sectional view taken along line XXIIIB of FIG. 23A. In FIG. 23, reference numerals 501a and 501b denote ceramic substrates, and reference numeral 517 denotes a common electrode formed of a bulk material.
FIG. 24 illustrates a thermal head disclosed in Japanese Unexamined Patent Application Publication No. 10-151784, wherein FIG. 24A is a top view of the thermal head and FIG. 24B is a cross-sectional view taken along line XXIVB of FIG. 24A. In FIG. 24, reference numeral 602 denotes a metal substrate having a projection 603, and reference numerals 608 and 611 denote heating resistors.
FIG. 25 illustrates a conventional thermal head, wherein FIG. 25A is a top view of the thermal head and FIG. 25B is a cross-sectional view taken along line XXVB of FIG. 25A. In FIG. 25, reference numeral 701 denotes a substrate formed of single silicon crystal, and reference numeral 707 denotes a common electrode. Reference numeral 702 denotes a through-hole formed in the common electrode 707, wherein the inner surface of the through-hole is plated with a conductive metal 703. Reference numerals 704 and 705 denote heating resistors.
FIG. 26 illustrates another conventional thermal head, wherein FIG. 26A is a top view of the thermal head and FIG. 26B is a cross-sectional view taken along line XXVIB of FIG. 26A. In FIG. 26, reference numerals 858, 854, 863 and 864 denote heating resistors, and reference numeral 852 denotes glaze glass.
In the case of the thermal head shown in FIG. 23, there is a difference in thermal expansion coefficient between the common electrode 517 and the substrate 501a or 501b, and the difference in thermal expansion coefficient can cause partial removal of the common electrode 517. Thus, performance degradation occurs as the thermal head is used for a long period of time.
In the case of the thermal head shown in FIG. 24, the substrate 602 is heated by a common current flowing through the projection 603 which is a part of the substrate 602. As a result, thermal interference occurs between the heating element 608 and the heating element 611. This makes it difficult to control the heating elements 608 and 611 independently of each other.
In the case of the thermal head shown in FIG. 25, a complicated process is needed to form the through-hole 702 through the substrate of single silicon crystal.
In the case of the thermal head shown in FIG. 26, if the heating resistors 853 and 854 are located very close to the heating resistors 863 and 864, interference due to heat storage in a partial glaze occurs because the heating resistors 853, 854, 863 and 864 are formed on the same partial glaze. The interference can cause the thermal head to become thermally uncontrollable. Although the above problem can be avoided by increasing the distance between two lines of heating resistors, the contact condition between the thermal head and a platen roller (not shown) which urges print paper against the thermal head becomes poor. To improve the contact condition, it is needed to increase the diameter of the platen roller or increase the force applied to the platen roller.
In the conventional thermal head, the thermal head heater is continuously energized until a needed intensity of color is obtained each time a line is printed. In this technique, the printed color intensity increases as the temperature (amount of heat) of the thermal head increases.
When respective colors of yellow, magenta, and cyan are printed on paper, no color appears during a particular period after turning on the thermal head, wherein the period in which no color appears varies depending upon the color. If, each time a line is printed, the thermal head is energized during a period in which no color appears plus a period needed to obtain a desired intensity of color, a long printing time is needed, that is, the printing speed becomes low.
One technique to avoid the above problem is to preheat paper using a preheating heater (preheater) to a temperature which is very close to but lower than a minimum temperature needed to develop a color. In this technique, a printing thermal head heater is used to further heat the paper to develop a color. Thus, color is developed with no delay and thus the problem of the reduction in the printing speed is avoided.
Although most of the heat generated by the preheater is consumed to preheat print paper, the heat is partially accumulated in the preheater and parts in the vicinity of the preheater. As a result, when the same amount of heat is generated by the thermal head heater over the entire surface of paper, the printed color intensity is low at a line (first line) at which the printing is started and the printed color intensity increases as the printing operation advances toward a final line as shown in FIG. 27. That is, nonuniformity in printed color density occurs.
Furthermore, the preheating can cause a color to be developed in a white-data area in which any color should not appear. In the case where the intensity specified by print data varies across print paper, the preheating can cause a deviation in color intensity from the specified intensity.
In the case where printing is performed using both a printing thermal head and a preheating heater, if the printing and the preheating are performed at the same time, a high-capacity power supply capable of supplying a high current with a high voltage is needed to drive the thermal head, and a complicated configuration is needed.
In view of the above, it is an object of the present invention to provide a thermal head which is formed of a material which does not cause removal, which can be produced without needing complicated processing, and which has less thermal interference.
It is another object of the present invention to provide a controller for controlling a thermal head, capable of controlling the thermal head without producing nonuniformity in color intensity caused by preheating using a preheater and without needing a high-voltage/high-current power supply for driving the thermal head.
According to an aspect of the present invention, there is provided a thermal head comprising: a metal substrate; an insulating layer formed on the surface of the metal substrate; a plurality of heating elements disposed on the surface of the insulating layer, the heating elements being arranged with a predetermined pitch along a plurality of lines in a main scanning direction, the plurality of lines being spaced from each other in a paper feeding direction perpendicular to the main scanning direction; and a heat radiating element projecting from the surface of the metal substrate to the side where the insulating layer is disposed. Note that the heat radiating element does not include a member serving as a path for supplying a current to the heating elements.
In this structure, although most of heat generated by the respective heating elements is transferred to an ink ribbon or print paper, residual partial heat is absorbed by heat radiating means via the insulating layer and radiated into the atmosphere. This suppresses thermal interference among the heating elements.
In this thermal head according to the present invention, a part, in contact with one line of the heating elements, of the insulating layer and a part, in contact with a directly adjacent line of the heating elements, of the insulating layer may be separated from each other by the heat radiating element.
This further suppresses thermal interference among the heating elements.
In this thermal head according to the present invention, preferably, the heat radiating element is disposed at least in a part of a region between the metal substrate and a gap between one line of the heating elements and an adjacent line of the heating elements, and a part, in contact with one line of the heating elements, of the insulating layer and a part, in contact with a directly adjacent line of the heating elements, of the insulating layer are connected to each other in a region in contact with the gap so that heat can be conducted therebetween.
In this structure, it is possible to prevent print paper preheated by one of two lines of the heating elements from being cooled when it passes over the intermediate part between the two lines of the heating elements.
In this thermal head according to the present invention, the heat radiating element may be formed integrally with the metal substrate.
This structure allows heat absorbed by the heat radiating element to be transferred more easily into the substrate and radiated. As a result, the effective radiating area increases, and thus a greater amount of heat is radiated into the atmosphere.
In the thermal head according to the present invention, portions, in contact with the heating elements, of the insulating layer may protrude in a direction toward the heating elements.
This structure ensures that heat is transferred to print paper in a more reliable fashion.
In the thermal head according to the present invention, the heating elements may be disposed such that the location, in the main scanning direction, of each heating element is coincident with the location of one of heating elements arranged in an adjacent line.
In this structure, it is possible to simultaneously generate heat in two lines of heating elements, and thus an increase in the printing speed is achieved.
In the thermal head according to the present invention, the heating elements may be disposed such that the location, in the main scanning direction, of each heating element is shifted by xc2xd pitch relative to the location of one of heating elements arranged in an adjacent line.
In this structure, a greater dot density can be achieved, and thus higher-precision printing becomes possible.
In the thermal head according to the present invention, the metal substrate may include a fin formed on a side opposite to the side on which the insulating layer is formed.
In this structure, a greater heat radiating area is provided to radiate a greater amount of heat into the atmosphere.
In the thermal head according to the present invention, two conductor patterns for supplying a current to each heating element to generate heat are connected to each heating element, on the side opposite to the insulating layer.
According to another aspect of the present invention, there is provided a thermal head controller for controlling a thermal head for use in a printer, the thermal head serving to form an image with at least one color on print paper, the thermal head including a preheating heater and a printing heater, the thermal head controller comprising: preheating control means for controlling preheating of each line performed by the preheating heater; and amount-of-heat correction means for correcting the amount of heat generated by the preheating heater for each line such that the effective amount of preheating heat is maintained substantially constant over all lines.
In this construction, even if heat generated by the preheating heater is stored in a part near the preheating heater, nonuniformity in color intensity does not occur because the effective amount of heat given to each line during the preheating process is maintained substantially constant.
The thermal head controller according to the present invention may further comprise temperature detection means, and the amount-of-heat correction means may correct the amount of heat in accordance with a temperature value detected by the temperature detection means.
The temperature detection means may include one of or both of an inside-of-printer temperature detector and a preheater temperature detector.
In the thermal head controller according to the present invention, the amount-of-heat correction means may correct the amount of heat depending upon a printing mode, a temperature inside the printer, a preheater temperature, and a line number.
In this case, at the beginning of a printing operation for one surface of paper, the amount-of-heat correction means may select data to be used depending upon the printing mode, the temperature inside the printer, and the preheater temperature, and the amount-of-heat correction means may determine, from the data, an amount of correction of heat depending upon the line number and correct the amount of heat by the determined amount of correction during the printing operation for the one surface of paper.
This construction makes it possible to correct the amount of heat generated in the preheating process for each line depending upon the amount of heat stored in parts other than a part which should be preheated by the preheating heater.
In the thermal head controller according to the present invention, the amount-of-heat correction means may correct the amount of heat depending upon a printing mode, a temperature inside the printer, and a preheater temperature.
In this case, at the beginning of a printing operation for one surface of paper, the amount-of-heat correction means may select data to be used depending upon the printing mode and the temperature inside the printer, and the amount-of-heat correction means may determine, from the data, an amount of correction of heat depending upon the preheater temperature and correct the amount of heat by the determined amount of correction during the printing operation for the one surface of paper.
This construction makes it possible to correct the amount of heat generated in the preheating process for a particular preheater temperature depending upon the amount of heat stored in parts other than a part which should be preheated by the preheating heater.
In the thermal head controller according to the present invention, preferably, the preheating control means energizes the preheating heater in a period in which printing is not performed by the printing heater and which is within a printing cycle.
In this configuration, the printing heater and the preheating heater are not energized at the same time during the same printing cycle, and thus preheating and printing can be performed without needing a special high-voltage and high-current power supply for driving the thermal head.
In the thermal head controller according to the present invention, the preheating control means may include: a first gate circuit for generating, in response to starting of a printing cycle for each line, a first signal indicating an energization start time of the preheating heater; a second gate circuit for generating a second signal indicating an energization end time at which the energizing of the preheating heater should be ended before starting energizing of the printing heater; and a third gate circuit for generating a preheating signal in accordance with the first signal and the second signal such that the preheating signal is activated over a period from the energization start time of the preheating heater to the energization end time, wherein the energization end time is changed by the amount-of-heat correction means.
In this construction, the period during which the preheater is energized is specified by the first signal generated by the first gate circuit and the second signal generated by the second gate circuit. Herein, if the first and second signals are set so that the timings of the energization start time and the energization end time of the preheating heater become earlier the energization start time of the printing heater, it becomes possible to energize, in each printing cycle, the preheating heater during a period in which printing is not performed by the printing heater.
Furthermore, the second gate circuit may include a counter which counts a predetermined clock signal and outputs a signal which determines the end time when the counted number of pulses reaches a value predetermined as a preset value.
With this construction, if the counter is set such that before the energizing of the printing heater is started, the count value will reach a value corresponding to the preheat end time, the preheating using the preheater can be ended when the count value reaches the preset value. This makes it possible to end the energization of the preheating heater before starting the energization of the printing heater in the same printing cycle.
According to still another aspect of the present invention, there is provided a thermal head controller for controlling a thermal head for use in a printer, the thermal head serving to form an image with one or more colors on print paper, the thermal head including a preheating heater and a printing heater, the thermal head controller comprising: signal generating means for generating a control pulse signal serving as a reference signal according to which the energizing of the printing heater is controlled; and preheating control means for controlling the energizing of the preheating heater by means of counting the control pulse signal.
In this construction, the energization start time of the preheating heater is controlled by means of counting the control pulse signal which is used to control the energization of the printing heater. This makes it possible to control the energization start time of the preheating heater without resulting in an increase in complexity of the circuit. Furthermore, by setting the control pulse signal to have a proper pattern, it is possible to control both the energization of the preheating heater and the energization of the printing heater in a highly effective fashion thereby increasing the printing speed using a simple configuration.
In the thermal head controller according to the present invention, the preheating control means may include: a counter which counts pulses of the control pulse signal and outputs a predetermined signal when the counted number of pulses reaches a value predetermined as a preset value; a flip flop for latching predetermined data and outputting it in response to the predetermined signal serving as a trigger signal; and a switch connected in series to the preheating heater, for controlling the energizing of the preheating heater in accordance with a signal output from the flip flop.
In this construction, when the count value of the control pulse signal reaches the preset value, the counter outputs a predetermined signal in the form of, for example, a pulse. In response to this predetermined signal serving as a trigger signal, the flip flop outputs predetermined data. That is, when the count value reaches the preset value, the flip flop produces a transition of its output thereby indicating that the count value has reached the preset value.
In the thermal head controller according to the present invention, before starting preheating using the preheating heater, the counter inputs a value as a preset value indicating a time at which the preheating should be started.